A fluorescent layer obtained by adding a small amount of activator such as Mn or Cu to ZnS or ZnSe is provided between a transparent electrode and opposing electrode and a specified voltage is applied across both electrodes. Thereby, said fluorescent layer emits light. A flat light emitting panel utilizing such fluorescence phenomenon is called EL panel.
Such EL panel can be classified into a dispersive type and thin film type in accordance with a method for forming the fluorescent layer or into a DC type and AC type in accordance with a driving method.
Said dispersive type fluorescent layer can be formed by obtaining a paste through dispersion of fine particle powder adding a small amount of Mn or Cu to ZnS or ZnSe into the solution of organic binder, and then coating it onto transparent electrodes by screen printing or with a doctor knife. Meanwhile, a thin film type fluorescent layer can be formed utilizing a thin film forming technology such as vacuum-deposition or sputtering method etc. Said DC type uses a DC power supply as a driving source, while AC type utilizes a AC power supply as a driving source. The present invention is directed to dispersive EL.
FIG. 1 shows a sectional view of a conventional dispersive EL panel. A transparent electrode 2 is formed on a transparent substrate 1 such as a glass plate and a fluorescent layer 3 is coated on such transparent electrode 2. An opposing electrode 4 is composed of a metal thin film formed by vacuum deposition or sputtering of aluminum and it is opposed to the transparent electrode 2 through the fluorescent layer 3.
When a DC voltage is applied across the transparent electrode 2 and the opposing electrode 4, a heavy current flows in the initial stage but light is not emitted. As the voltage is gradually increased, the current decreases on the contrary and light is emitted at a certain voltage. This process is called forming and when forming once occurs, the light is emitted in a peculiar color with a minute current.
However, this dispersive EL has following disadvantages. FIG. 2 shows an enlarged sectional view of the junction area of the fluorescent layer 3 and opposing electrode 4 of such dispersive EL. The opposing electrode 4 is often not in even contact with the surface of fluorescent layer 3. As described above, the fluorescent powder paste is coated and dried up, in the dispersive EL, in order to form the fluorescent layer 3. Therefore, the surface is considerably uneven because of bubbles in the fluoresent powder paste or roughness of particles due to aggregation of fluorescent powder 5. On the other hand, the opposing electrode 4 is composed of a metal thin film formed by vacuum-deposition etc. and therefore it is inferior in flexibility and close-contactness and clearances 6 are generated to and fro between the fluorescent layer 3 and opposing electrode 4. When such clearances 6 exist, the contact area between the fluorescent layer 3 and opposing electrode 4 becomes small and a resistance value between the transparent electrode 2 and opposing electrode 4 becomes large. As a result, a forming end voltage becomes high and a drive voltage is inevitably increased. Moreover, as described above, if such clearances 6 exist as described above, additional disadvantages are generated, namely the corresponding area does not emit the light and brightness is as much lowered.
In order to improve close contact between the fluorescent layer and opposing electrode, it has been proposed that a conductive resin bonding layer obtained by additionally mixing conductive fine particles such as carbon to the resin in a hot melt be provided between said fluorescent layer and opposing electrode. Here, said conductive resin adhesive contains a large amount of a thermosetting resin (binder component) in the adhesive layer in order to increase adhesivity. The conductive fine particles are mainly chained in order to result in conductivity and the spaces between such chaining structure are filled with resin. Therefore, even a conductive resin adhesive layer has an electric resistance value as high as several hundred ohms to several thousand ohms. If the conductive layer's resistance is high as described above the forming step does not proceed uniformly and the forming speed is different at the edge portion of the light emitting surface and the central area. Such difference readily causes flucturation in light emission, uniform light emission cannot be obtained in a wider area and a high driving voltage is required.
In addition, a conductive resin adhesive layer is attached to the fluorescent layer after said adhesive layer is softened and melted by heating. In such a case, viscosity is high and therefore it is difficult to enter the fine clearance at the surface of the fluorescent layer, resulting in fluctuation of light emission.